Among other difficult things, these astronomers have attempted to measure the distance of the sun, moon, and stars from our earth. Moreover, they have tried to ascertain the exact size of these celestial lights, and they have, to a considerable extent, been successful in their efforts. By their complicated calculations, the men who study the stars can tell the exact day, hour, and minute when certain events will happen, such as an eclipse of the sun or of the moon. Now, about the year 1768 the attention of the scientific world was eagerly turned to an event which was to take place in the following year. This was the passage of the planet Venus across the face of the sun. Astronomers term this the _Transit of Venus_. It happens very seldom: it occurred in 1769, but not again till 1874, and 1882. By observing this passage--this transit--of Venus across the sun from different parts of our earth, it was hoped that such information could be obtained as would enable us to measure not only the distance of the sun from the earth with greater accuracy than heretofore, but also the extent of the whole host of stars that move with our earth around the sun and form what is called our Solar System. An opportunity occurring so seldom was not to be lost. Learned men were sent to all parts of the world to observe the event. Among others, Captain Cook was sent to the south seas--there, among the far-off coral isles, to note the passage of a little star across the sun's face--an apparently trifling, though in reality important, event in the history of science. So much for the object of Cook's first voyage. Let us now turn to the details thereof. The vessel chosen by him for his long and dangerous voyage to unknown seas was a small one of only 370 tons burden. It was named the _Endeavour_. The crew consisted of forty-one seamen, twelve marines, and nine servants--these, with the officers and the scientific men of the expedition, made up a body of eighty-five persons. The scientific men above mentioned were, Mr Green, an astronomer; Mr Banks, a naturalist, who afterwards became Sir Joseph Banks and a celebrated man; Doctor Solander, who was also a naturalist; and two draughtsmen, one of whom was skilled in drawing objects of natural history, the other in taking views of scenery. The _Endeavour_ was victualled for a cruise of eighteen months. She was a three-masted vessel of the barque rig, and carried twenty-two large guns, besides a store of small arms,--for the region of the world to which they were bound was inhabited by savages, against whom they might find it necessary to defend themselves. When all was ready, Captain Cook hoisted his flag, and spread his sails, and, on the 26th of August 1768, the voyage began--England soon dropped out of sight astern, and ere long the blue sky above and the blue sea below were all that remained for the eyes of the navigators to rest upon.